In recent years, in the automotive field, in order to improve fuel economy and slash CO2 emissions, reduction of the weight of the vehicle body has been demanded. To improve crash safety, increasing the strength of the members of the car body has also been demanded. In order to satisfy these demands, it is effective to use high strength steel sheet for members of the vehicle body and various parts.
Further, from the viewpoint of increasing rust prevention of the car body, it is necessary to construct members from steel sheets excellent in corrosion resistance. It is widely known that galvanized steel sheets have good corrosion resistance. From the viewpoint of reducing the weight and raising the strength, in galvanized steel sheets used for automobiles, galvanized high strength steel sheets using high strength steel sheets for the plated sheets are being used.
In the assembly of automobile bodies and attachment of parts etc., spot welding is mainly used. If spot welding galvanized high strength steel sheets, cracking may occur in the sheet thickness direction from the outer surfaces of the steel sheets in contact with the electrodes for spot welding use.
FIG. 1 shows an outline of cracking in a spot welded location when spot welding galvanized high strength steel sheets. FIG. 1 is a cross-section in the sheet thickness direction. When spot welding galvanized high strength steel sheets 1, it is known that cracking 3 advancing from a surface of a steel sheet 1 in contact with an electrode toward the melted and solidified part 2 (nugget) (below, referred to as “cracking right below an electrode”), cracking 5 advancing from a portion of a steel sheet 1 in contact with a shoulder portion of an electrode to a heat affected zone 4 (below, referred to as “cracking at a shoulder”), and cracking 6 advancing from an outside of a portion where an electrode and a steel sheet 1 come into contact to a heat affected zone 4 (below, referred to as “cracking outside of an electrode”) occur.
Below, when it is not particularly necessary to differentiate them, “cracking right below an electrode”, “cracking at a shoulder”, and “cracking outside of an electrode” will be collectively referred to as “external cracking”. Further, “cracking at a shoulder” and “cracking outside of an electrode” will be collectively referred to as “cracking near an outer circumference of a weld”.
Such cracking is said to be cracking due to so-called “liquid metal embrittlement”. That is, it is said that by applying the electrode pressing force and the tensile stress due to thermal expansion and contraction of the steel sheets to the weld zone, the molten galvanized metal invades the grain boundaries of the steel sheets and decreases the intergranular strength.
In automobile bodies, if the cracking in a welded location is remarkable, the strength of the joint decreases. There are known techniques for suppressing cracking at a welded location by methods of controlling the chemical composition and structure of the steel sheets.
For example, PLT 1 discloses to adjust the chemical composition of the steel sheets, render the austenite phase generated during the spot welding to fine crystal grains, and complicatedly interpose them with crystal grains of other phases in the metal structure so as to thereby make the paths for diffusion and penetration of molten zinc to the crystal grain boundaries complicated and make it difficult for molten zinc to penetrate and thus prevent liquid metal embrittlement cracking at the time of welding.
Further, PLT 2 teaches that by merely making the crystal grain boundaries more complicated by controlling the structures of the steel sheets, it is not possible to sufficiently suppress the occurrence of cracking at a welded location. It discloses to adjust the chemical compositions of the steel sheets, make the intergranular penetration depths of the hot rolled steel sheets 5 μm or less, and electroplate by Fe cold rolled steel sheets before hot dip galvannealing so as to make the intergranular penetration depths of the hot dip galvannealed steel sheets 5 μm or less and thereby suppress the occurrence of cracking at a welded location of the hot dip galvannealed steel sheets.